What is the best initial test for a patient suspected of having lupus antiphospholipid syndrome?

Best Test for Lupus Antiphospholipid Syndrome

For any patient suspected of having antiphospholipid syndrome, you must order all three core tests simultaneously: lupus anticoagulant (LA), anticardiolipin antibodies (aCL) IgG and IgM, and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG and IgM—there is no single "best" test, as comprehensive testing is required for accurate diagnosis. 1, 2

The Complete Testing Panel

The International Society on Thrombosis and Haemostasis explicitly recommends concurrent testing for all three antibody types on the same sample 1, 2. This approach is mandatory because:

• Lupus anticoagulant (LA) is the strongest single predictor of thrombotic risk, particularly when combined with positive ELISA tests 2
• Triple positivity (LA + aCL + aβ2GPI) carries a 33-fold increased risk of thrombosis-related events compared to single positivity 3
• Omitting either component can miss up to 55% of triple-positive patients, leading to dangerous underdiagnosis 4

How to Perform Lupus Anticoagulant Testing

LA testing requires a specific 3-step methodology 1, 4:

1. Screening test: Use both dilute Russell's viper venom time (dRVVT) and a sensitive aPTT (with low phospholipids and silica as activator) in parallel 1, 4
2. Mixing study: Mix patient plasma 1:1 with pooled normal plasma to distinguish factor deficiency from inhibitor 1
3. Confirmatory test: Add excess phospholipids to demonstrate phospholipid-dependent inhibition 1

Critical caveat: Blood must be collected in 0.109 M sodium citrate with double centrifugation, ideally before starting anticoagulation therapy 1. LA testing during anticoagulation is unreliable and can produce false results 2, 4.

How to Perform Solid-Phase Antibody Testing

For aCL and aβ2GPI antibodies 1:

• Measure both IgG and IgM isotypes using ELISA or automated platforms 1
• Results must exceed the 99th percentile of normal controls to be considered positive 1
• The IgG isotype is clinically more relevant than IgM 1, 4
• aCL antibodies must be β2-glycoprotein I-dependent to avoid detecting non-pathogenic antibodies associated with infections 1

Confirmation Testing is Mandatory

Any positive result must be confirmed with repeat testing at least 12 weeks after the initial test 1, 2, 5. This 12-week interval distinguishes persistent pathogenic antibodies from transient positivity that does not indicate APS 1, 5. The same antibodies must be positive on both occasions 1.

Risk Stratification Based on Results

The antibody profile determines thrombotic risk 2, 4, 3:

• Triple positive (LA + aCL + aβ2GPI of same isotype): Highest risk, warrants most aggressive management 2, 4, 3
• Double positive (aCL and aβ2GPI with concordant isotype): Significantly increased confidence in APS diagnosis 1, 4
• Single positive LA: Lower thrombotic risk than triple positivity, but still clinically significant 4
• Single positive IgM only: Least clinically relevant 1, 4

Research demonstrates that triple positivity confers a 23-fold increased risk compared to double positivity and even higher risk compared to single positivity 6.

Patient Selection for Testing

Testing should be limited to patients with significant probability of APS 1:

High-priority patients (test immediately) 1:

• Unprovoked venous thromboembolism in patients <50 years
• Unexplained arterial thrombosis in young patients
• Thrombosis at unusual sites
• Late pregnancy loss
• Any thrombosis or pregnancy morbidity in patients with autoimmune diseases (SLE, rheumatoid arthritis, autoimmune thrombocytopenia)

Moderate-priority patients 1:

• Unexplained prolonged aPTT in asymptomatic subjects
• Recurrent spontaneous early pregnancy loss
• Provoked VTE in young patients

Low-priority patients (testing discouraged) 1:

• Venous or arterial thromboembolism in elderly patients
• Asymptomatic screening

Common Pitfalls to Avoid

Anticoagulation interference 2, 4:

• Ideally, test 1-2 weeks after discontinuing vitamin K antagonists (with or without LMWH bridging) 1
• For patients on DOACs, use pretest DOAC removal procedures 1, 4
• For patients on VKAs where testing cannot be delayed, Taipan snake venom time/ecarin time can be used, though sensitivity is not 100% 1, 4

Acute phase interference 4:

• Factor VIII increases during pregnancy can mask LA by shortening aPTT 5
• Antibody levels may decrease during acute thrombosis due to antibody deposition at the thrombotic site 5
• Consider retesting if initial results were obtained during these conditions 5

Interpretation errors 1:

• Values within 10% of the cutoff may be affected by assay imprecision—retest equivocal results 1, 5
• Results must be interpreted in clinical context with close laboratory-clinician collaboration 1, 2, 4